PHOTOCONDUCTIVE SAMPLING WITH AN INTEGRATED SOURCE FOLLOWER AMPLIFIER

A hybrid, optoelectronic sampling circuit based on a photoconductive s witch and a junction-field-effect-transistor (JFET) source follower/am plifier has been demonstrated to have picosecond response, high-sensit ivity, absolute-voltage capability, and a very high impedance. The dis tributed capacitance of the electrical measurement system is reduced t o the gate input capacitance of the JFET, and the conventional photoco nductive current measurement is transferred into an absolute voltage m easurement. Gating measurements with an improvement of 150 times in ou tput voltage over unamplified photoconductive gates have been made usi ng only 10 mu W of average optical power. The effective on-state resis tance of the photogate has also been increased by more than 150 times, indicating that a photoconductive probe with very low invasiveness ma y be produced. (C) 1996 American Institute of Physics.